Metal nanoparticles of about 2 nm to 100 nm in diameter have characteristics different from those of bulk metal, in terms of optical properties, magnetic properties, heat properties, electrical properties, etc. Therefore, such nanoparticles are expected to be applied in various technical fields. For example, taking advantage of the properties that a smaller particle size has a larger surface area and induces a reduction in the melting point, research on the production of electronic circuits comprising a fine metal wire on substrates using fine-wire printing ink containing metal nanoparticles has proceeded.
The fine-wire printing ink uses, as an ink material, a dispersion containing metal nanoparticles, whose surface is protected by an organic substance, and a circuit pattern is printed on a substrate by using a fine-wire printing technique, and heated at a low temperature to thereby remove the organic substance from the surface of the metal nanoparticles, and to form metal bonds between the metal nanoparticles. In particular, when metal nanoparticles having a diameter of 10 nm or less are used, the melting point is significantly reduced. Thus, a fine metal wire having high thermal conductivity and electrical conductivity can be formed.
Silver nanoparticles are mainly used as a fine-wire printing ink material (see PTL 1). However, when the silver nanoparticles disclosed in PTL 1 are used, the so-called “migration phenomenon” is likely to occur. Specifically, silver in the fine wire is oxidized and thereby ionized, and moves on the insulating material of the substrate, thereby inducing a short circuit. Further, use of gold has also been examined. Gold is preferable because it is less likely to induce the migration phenomenon; however, there is the problem that it is expensive.
Accordingly, as the metal used as a fine-wire printing ink material, copper has attracted attention because it is less likely than silver to induce the migration phenomenon and its cost is relatively low.
Bulk copper, which is conventionally used as metal wire, has drawbacks in that, for example, it is easily oxidized, thereby reducing conductivity, and has a high sintering temperature. In contrast, copper nanoparticles have a lower sintering temperature than bulk copper, and are expected to be a material that can form fine metal wire on a substrate vulnerable to heat, such as paper or plastic.
However, copper nanoparticles are aggregated more easily than other metal nanoparticles, such as gold and silver, and the aggregate has a particle diameter of several tens of nm to several hundreds of nm; therefore, it is difficult to synthesize monodisperse copper nanoparticles having an average particle diameter of 10 nm or less, which are particularly useful as an ink material. For example, NPL 1 teaches that crystalline copper nanoparticles having a particle size of around 50 nm are obtained by refluxing a copper component in an ethylene glycol solvent for 2 hours. Further, NPL 2 teaches that copper-nickel composite particles having a particle size of several hundreds of nm are obtained by rapidly heating a solution in which a copper compound, a nickel compound, and a base are dissolved in ethylene glycol to the boiling point using a heater. In particular, NPL 2 teaches that copper nanoparticles having a particle size of several hundreds of nm are obtained at a boiling point of around 165° C. in a state in which hydrated water of a copper compound and a nickel compound is contained.
Copper nanoparticles having an average particle diameter of 10 nm or less that undergo a remarkable reduction in the melting point are desired as a nanoink material that can form a fine metal wire on a substrate vulnerable to heat, such as paper or plastic; however, there have been no copper nanoparticles that can be sintered at a low temperature within such a temperature region. This is because copper nanoparticles having an average particle diameter of 10 nm or less are highly reactive and thus unstable, and the oxidation and aggregation of the copper nanoparticles easily occur. It is difficult to stably preserve such copper nanoparticles even immediately after the copper nanoparticles are obtained. Therefore, microscopic copper nanoparticles having an average particle diameter of 10 nm are produced using, as a protective agent, a polymer or the like that strongly binds to the copper surface. However, there is a problem that the protective agent cannot be completely removed during low-temperature heating, leading to a reduction in the electrical conductivity of the fine metal wire.
As a method for producing copper nanoparticles having a small average particle diameter, PTL 2 discloses a method for producing metal nanoparticles by reacting a reducing agent with a solution containing an organic acid metal salt and an amine compound. Further, PTL 3 discloses a method for producing copper nanoparticles by reacting a reducing agent with a solution comprising an organic acid copper salt and C8-16 monoamine. PTL 2 discloses a method for producing copper nanoparticles of about 5 nm, and PTL 3 discloses a method for producing copper nanoparticles having an average particle diameter of 10 nm or less and having a uniform particle size distribution.
However, it is difficult to decompose and remove the amines having 8 or more carbon atoms shown in PTL 2 and PTL 3 during low-temperature heating. For example, the organic amine protective layer on the surface of the copper nanoparticles cannot be removed at 150° C. or less. In PTL 2 and PTL 3, copper nanoparticles with an organic amine protective layer having 8 or more carbon atoms were actually produced. It is not disclosed that the copper nanoparticles can be sintered at a low temperature of 150° C. or less, and that the amine protective layer of the copper nanoparticles can be removed. Thus, there is demand for a protective layer comprising a short-chain amine that allows the removal of the amine protective layer from the surface of the copper nanoparticles even by low-temperature sintering at 150° C. or less. However, such a short-chain amine has low protective power, thus causing problems that the coarsening, aggregation, and oxidation of the copper nanoparticles are likely to occur, and that stable copper nanoparticles having an average particle diameter of 6 nm or less cannot be obtained.
Copper nanoparticles having an average particle diameter of 10 nm or less have a large surface area and are easily oxidized; however, PTL 2 and PTL 3 merely disclose copper nanoparticles having an average particle diameter of 10 nm or less, which is observed with an electron microscope. Further, PTL 2 and PTL 3 do not provide any data showing that the obtained copper nanoparticles are not oxidized.
Moreover, PTL 4 discloses a method for storing a nanoparticle dispersion, the method comprising storing, at 10° C. or less, a nanoparticle dispersion in which copper nanoparticles or copper oxide nanoparticles each coated with a protective agent are dispersed in a solvent. Paragraph [0015] of PTL 4 states that the coating amount of the protective agent is 30 parts by mass or more and 150 parts by mass or less based on 100 parts by mass of each copper nanoparticle or copper oxide nanoparticle. However, in terms of simplicity and the convenience of transport, there is demand for a method that can preserve and transport copper nanoparticles at room temperature (10° C. or more), and that can preserve them for a long period of time.
From the above viewpoint, copper nanoparticles used as a conductive copper ink material that can be sintered at a low temperature are required to satisfy the following requirements:
(1) they are highly dispersible copper nanoparticles having an average particle diameter of 10 nm or less that undergo a remarkable reduction in the melting point;
(2) the protective layer of the copper nanoparticles can be removed during low-temperature sintering at 150° C. or less; and
(3) the oxidation of the copper nanoparticles is suppressed.
In addition, in terms of production scale-up and low cost for industrialization, the copper nanoparticles are required to satisfy the following requirement:
(4) the copper nanoparticles can be stably preserved at room temperature for a long period of time, and can be transported.
However, at present, copper nanoparticles, copper nanoparticle dispersion, copper nanoink, and methods for preserving copper nanoparticles do not satisfies these requirements.